[PATCH v7 2/5] mm: hugetlb: replace hugetlb_free_vmemmap_enabled with a static_key

[Muchun Song <songmuchun@xxxxxxxxxxxxx>]

The page_fixed_fake_head() is used throughout memory management and
the conditional check requires checking a global variable, although
the overhead of this check may be small, it increases when the memory
cache comes under pressure. Also, the global variable will not be
modified after system boot, so it is very appropriate to use static
key machanism.

Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
Reviewed-by: Barry Song <song.bao.hua@xxxxxxxxxxxxx>
---
 include/linux/hugetlb.h    |  6 ------
 include/linux/page-flags.h | 16 ++++++++++++++--
 mm/hugetlb_vmemmap.c       | 12 ++++++------
 mm/memory_hotplug.c        |  2 +-
 4 files changed, 21 insertions(+), 15 deletions(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 44c2ab0dfa59..27a2adff0db7 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -1077,12 +1077,6 @@ static inline void set_huge_swap_pte_at(struct mm_struct *mm, unsigned long addr
 }
 #endif	/* CONFIG_HUGETLB_PAGE */
 
-#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
-extern bool hugetlb_free_vmemmap_enabled;
-#else
-#define hugetlb_free_vmemmap_enabled	false
-#endif
-
 static inline spinlock_t *huge_pte_lock(struct hstate *h,
 					struct mm_struct *mm, pte_t *pte)
 {
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 7cd386538d0c..26e540fd3393 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -185,7 +185,14 @@ enum pageflags {
 #ifndef __GENERATING_BOUNDS_H
 
 #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
-extern bool hugetlb_free_vmemmap_enabled;
+DECLARE_STATIC_KEY_MAYBE(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
+			 hugetlb_free_vmemmap_enabled_key);
+
+static __always_inline bool hugetlb_free_vmemmap_enabled(void)
+{
+	return static_branch_maybe(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
+				   &hugetlb_free_vmemmap_enabled_key);
+}
 
 /*
  * If the feature of freeing some vmemmap pages associated with each HugeTLB
@@ -205,7 +212,7 @@ extern bool hugetlb_free_vmemmap_enabled;
  */
 static __always_inline const struct page *page_fixed_fake_head(const struct page *page)
 {
-	if (!hugetlb_free_vmemmap_enabled)
+	if (!hugetlb_free_vmemmap_enabled())
 		return page;
 
 	/*
@@ -233,6 +240,11 @@ static inline const struct page *page_fixed_fake_head(const struct page *page)
 {
 	return page;
 }
+
+static inline bool hugetlb_free_vmemmap_enabled(void)
+{
+	return false;
+}
 #endif
 
 static __always_inline int page_is_fake_head(struct page *page)
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 4977f5a520c2..791626983c2e 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -188,9 +188,9 @@
 #define RESERVE_VMEMMAP_NR		1U
 #define RESERVE_VMEMMAP_SIZE		(RESERVE_VMEMMAP_NR << PAGE_SHIFT)
 
-bool hugetlb_free_vmemmap_enabled __read_mostly =
-	IS_ENABLED(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON);
-EXPORT_SYMBOL(hugetlb_free_vmemmap_enabled);
+DEFINE_STATIC_KEY_MAYBE(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
+			hugetlb_free_vmemmap_enabled_key);
+EXPORT_SYMBOL(hugetlb_free_vmemmap_enabled_key);
 
 static int __init early_hugetlb_free_vmemmap_param(char *buf)
 {
@@ -204,9 +204,9 @@ static int __init early_hugetlb_free_vmemmap_param(char *buf)
 		return -EINVAL;
 
 	if (!strcmp(buf, "on"))
-		hugetlb_free_vmemmap_enabled = true;
+		static_branch_enable(&hugetlb_free_vmemmap_enabled_key);
 	else if (!strcmp(buf, "off"))
-		hugetlb_free_vmemmap_enabled = false;
+		static_branch_disable(&hugetlb_free_vmemmap_enabled_key);
 	else
 		return -EINVAL;
 
@@ -284,7 +284,7 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
 	BUILD_BUG_ON(__NR_USED_SUBPAGE >=
 		     RESERVE_VMEMMAP_SIZE / sizeof(struct page));
 
-	if (!hugetlb_free_vmemmap_enabled)
+	if (!hugetlb_free_vmemmap_enabled())
 		return;
 
 	vmemmap_pages = (nr_pages * sizeof(struct page)) >> PAGE_SHIFT;
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 3de7933e5302..587a8fc61fc8 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1326,7 +1326,7 @@ bool mhp_supports_memmap_on_memory(unsigned long size)
 	 *       populate a single PMD.
 	 */
 	return memmap_on_memory &&
-	       !hugetlb_free_vmemmap_enabled &&
+	       !hugetlb_free_vmemmap_enabled() &&
 	       IS_ENABLED(CONFIG_MHP_MEMMAP_ON_MEMORY) &&
 	       size == memory_block_size_bytes() &&
 	       IS_ALIGNED(vmemmap_size, PMD_SIZE) &&
-- 
2.11.0